Color display systems often rely on three separate sources to produce three primary colors of light. The intensities of the three primary colors can then be varied and mixed to produce various different colors in a color image. The eye's perception of color is related to the response of three different types of cells in the retina. Each type of cell responds to a different portion of the electromagnetic spectrum.
For the specific purpose of display or projection systems the best wavelength for “blue” light is about 450 nm (in vacuum). Such light is actually perceived by the human eye as a purplish-blue color as opposed to a pure blue. “Pure” blue light is typically characterized by a wavelength in the range of about 460 nm to about 480 nm. The reason for using 450 nm can be explained using the chromaticity diagram of FIG. 1. Given three colors that can be located on the chromaticity diagram, it is only possible to create by addition colors which are on the interior of a triangle created by placing corner points at the three colors. It is clear from FIG. 1 that a wavelength of 450 nm is ideal. A display system based on a wavelength of 470 nm would create a situation where a number of well saturated purples and red-purples are outside the triangle and, thus, not accessible to the display system.
A single laser which has output at the three colors of red, green and blue would be valuable for projection displays. Development of such lasers has been hampered by difficulties in producing blue light at sufficient power levels for use in a display. One current approach to generating high power levels of blue light is to use Nd:YAG lasers operating at 1064 nm. The output of the laser is frequency doubled with a nonlinear crystal to 532 nm. The frequency-doubled output then pumps an OPO. One of the OPO output wavelengths is then summed with the 532-nm light to create the blue. Thus 2 nonlinear steps in 2 separate crystals are required to produce blue light from infrared laser light. Since each step requires crystals, and has limited efficiency, the overall system is expensive and inefficient. Furthermore, Nd:YAG lasers require water-cooling and resonator structures, which add to the complexity, bulk and cost of the system.
For such lasers, and a number of other applications, it is desirable to have a single-frequency, pulsed laser source at an infrared wavelength suitable for conversion to blue light. High repetition rates are often needed, e.g., in image display systems because the time between pulses must be smaller than the duration of a modulation state of the light modulator that produces the image. For example, if the display presents data one pixel at a time, and if there are 1 million pixels, with an image refresh rate of 30 Hz, then a repetition rate of greater than 30 MHz is needed to avoid a situation where there is less than one pulse per pixel. If the display presents data one column of pixels at a time, then the laser repetition rate can be about 1000 times slower, or about 30 kHz, since the column of pixels contains 1000 pixels. However, even for systems where one column of pixels is presented at a time, it is desirable to have the pulse repetition rate greater than 1 MHz, so that very large pixel counts can be used at high refresh rates, and so that there are no pattern artifacts due to some pixel columns receiving more pulses of light than others.
Furthermore, a single frequency of oscillation for the infrared radiation is desirable so that conversion of the infrared light to blue light is efficient. If multiple frequencies are present, it may not be possible to optimize the frequency converter for each infrared frequency, and efficiency is reduced.
There is a need, therefore, for compact, efficient and inexpensive blue lasers, displays that use such lasers, and sources or methods of producing pulsed infrared light for use in such lasers.